x The Hydrosphere 169 



advances the surface cools rapidly, and since the water is 

 comparatively fresh it continues, in spite of its increasing 

 density, to float on the warmer sea-water below, and some- 

 times freezes, while at a depth of a few fathoms the tem- 

 perature of the salt water may be more than 45. The 

 heat of summer is conducted downward so slowly that the 

 highest temperature of the year is reached at the bottom 

 when the surface is at its coldest in January or February ; 

 the seasons at the bottom of Upper Loch Fyne or Loch 

 Goil, for example, being six months behind those at the 

 surface. In the far deeper basins of the fjords of Norway 

 seasonal changes of temperature penetrate to about 200 

 fathoms, but no farther. 



230. River and Sea- water. When a large swift river 

 flows directly into the sea it spreads out over the surface for 

 many miles, floating on the salt water, which it freshens 

 superficially. The form of the fresh stream may often be 

 traced by the contrast of its colour with the clear blue of the 

 ocean. Off the mouths of the Amazon and the Orinoco, for 

 example, muddy fresh water is found floating on the surface 

 of the sea several hundred miles from land. The Sun's heat 

 rapidly evaporates the floating fresh water, and salt from 

 below diffuses up and increases its density, thus enabling 

 it to mix with the mass of the ocean, a process assisted 

 by wind and waves. When rivers pour directly into a sea 

 affected by tides it may happen that the current of fresh 

 water is only slackened, but not reversed, by the rising tide. 

 In the Spey, which is the swiftest river in Britain, salt sea- 

 water is forced, like a dense fluid wedge, for a considerable 



'distance up the bed of the river by the rising tide, and 

 lifts the fresh stream to a higher level, so that perfectly 

 fresh water is found on the surface, separated by a brackish 

 layer a foot or two thick from the salt water below. The 

 salt wedge is withdrawn by the ebb-tide, and the river 

 current resumes its rapid flow to the sea. 3 Rivers which 

 enter the sea directly have little influence on the salinity 

 and temperature of the deeper layers of sea-water. 



231. Estuaries and Firths. In the La Plata, the 

 Thames, the Severn, the Forth, the Tay, the Garonne, and 



